1. Field of the Invention
This invention relates to a process for producing an insulating paper and, more particularly, it relates to a process for producing a polypropylene insulating paper having a high air-impermeability and excellent oil-resistance.
2. Description of the Prior Art
Heretofore, insulating papers useful in oil-filled electric devices and machinery, such as oil-filled cables and oil-filled condensers, have often been produced from polypropylene because of the low dielectric loss and low price of the polypropylene. Processes for producing such papers are disclosed, for example, in U.S. Pat. Nos. 3,532,800, 3,650,866, etc. However, the polypropylene paper produced in accordance with the process disclosed in U.S. Pat. No. 3,532,800 is made of a fiber mixture containing at least 50% by weight of microfibers having a diameter less than 10 microns and exhibits poor oil-resistance, and the polypropylene paper disclosed in U.S. Pat. No. 3,650,866 is constructed with fibers fabricated by the blowing method having a diameter of at least 0.5 denier and exhibits an unsatisfactory oil-resistance and air-permeability.